Heretofore, as a flush water supply device for supplying flush water to a flush water tank of a toilet, there has been known one type which comprises a small tank formed in a tub-like shape having a bottom with an opening to serve as a water compartment, and a float provided inside the small tank, wherein the float is adapted, when flush water held in the small tank is drained from the opening, to be lowered along with a lowering of a water level within the small tank, causing a water supply valve to be opened interlockingly with the float, as described, for example, the following Patent Document 1 (JP 2011-064009A).